Dying PSU's can also kill system hardware.
They dont provide enough voltage to your gear and it can kill stuff.
Had a dying PSU take out a video card before.
Always good to have a good bit of overhead. That way you aren't running it maxed all the time.
I have a UPS that shows me output wattage so I can get a good idea how much strain Im putting on it.
right now IM running quad SLI (Dual GTX 590s)... off a corsair AX1200.
So far Ive got plenty of overhead on games that can use the SLI.
Glad I didnt have to go up to a 1500w unit (those require a 20 amp line at the wall.. not the usual 15amp wire you find in most homes).
Never skimp on cheap power supplies.. it will bite you in the butt.
As far as the rails go.. even PSU's that claim multiple 20amp rails. Its still usually a single rail that they just divide off.
not a big deal. Just have to look at the amp ratting on them and what your card requires.
with your vid card, Id say that 600w is close.
the 560ti benched pulling about 135w idle and about 285 at load.
http://www.bit-tech.net/hardware/gra...i-1gb-review/8
you should have over 100-150 watts of overhead after you factor in your CPU/mobo/fans and drives.